Problem: $ B = \left[\begin{array}{rrr}1 & 2 & 1 \\ 1 & 1 & 2 \\ 2 & 0 & 1\end{array}\right]$ What is $ B^{-1}$ ?
Answer: $ = \left[\begin{array}{rrr}\frac{1}{5} & -\frac{2}{5} & \frac{3}{5} \\ \frac{3}{5} & -\frac{1}{5} & -\frac{1}{5} \\ -\frac{2}{5} & \frac{4}{5} & -\frac{1}{5}\end{array}\right]$